The present invention pertains to the art of bushings and, more specifically, to a bushing assembly particularly adapted for use in attaching a leaf spring of a vehicle suspension to a truck chassis. The invention is also directed to an improved method of installing a bushing assembly in a leaf spring suspension system or other support.
It is widely known in the vehicle art to utilize leaf springs in vehicle suspension systems. In particular, it is common to utilize leaf springs in rear suspension assemblies for both light and heavy-duty trucks. In such a suspension arrangement, the leaf spring is attached to a chassis of the truck at terminal support locations. More specifically, each end of the leaf spring is bent to form a spring eye which is attached to the chassis through a support member, generally constituted by either a bracket or a shackle. The leaf spring is preferably attached to the support member via a bushing assembly in order to provide rotational movement between the end of the leaf spring and the support member. Such rotation is desirable in order to provide enhanced suspension characteristics. Conversely, if the ends of the spring are supported in a fixed manner, such as when the spring/support connection freezes up, the leaf spring would essentially act as a cantilevered spring and thus provide deteriorated suspension characteristics.
In seeking to provide a high quality leaf spring support arrangement designed to maintain desirable suspension characteristics for prolong periods, improvements in the design of roller bushings utilized in connecting a leaf spring to a suspension support have been proposed. For example, U.S. Pat. No. 5,562,348 is directed to a compact roller bushing assembly, designed to rotatably connect an eye of a leaf spring to a chassis bracket, which generally requires less scheduled maintenance than prior known arrangements while sustaining relative rotation between the leaf spring and the truck chassis. Although this patented arrangement does provide an assembly which prevents the spring/support connection from freezing up and can be readily retrofitted to existing suspension systems, the arrangement does present some problems. For instance, the typical eye of a leaf spring often tends to be either too small or "out of round", i.e., not perfectly cylindrical, such that the tolerances between the outside diameter of the bushing assembly and the inside diameter of the spring eye can be extremely small. In this situation, a spring eye expander must be used to install the roller bushing assembly within the spring eye. This is the preferred installation method outlined in the '348 patent. However, such an installation method can place extra pressure on the roller bushing assembly, directly through the outer sleeve thereof, which can lead to various system problems. For instance, roller bearings, which are typically pre-mounted within a bushing sleeve, may be forced or shelled out of the bushing sleeve upon installation of the sleeve within the spring eye. In addition, the radial forces exerted on the bearing assembly can deform the sleeve and bearings such that the insertion of a chassis bracket pin therein is made difficult. In other words, the sleeve and bearing assembly become too small to allow the pin to be removed or reinstalled. Furthermore, the overall bearing and pin life can be significantly reduced if excessive pre-loads are applied to the bushing assembly. Such a pre-loading problem can occur, for example, when a bushing sleeve is forced into an out-of-round or small (low side of tolerance) spring eye thereby causing excessive radial forces to be applied to the sleeve which deforms the cylindrical shape of the inner surface of the sleeve. This deformation, in turn, can crush the bearing assembly and lead to premature failure thereof.
Therefore, this prior proposed roller bushing system may have practical limitations regarding its use in connection with spring eyes which are out-of-round to a certain degree at a low side of a tolerance fit range. In addition, considering that a rather extreme tolerance, i.e., in the order of 0.0005 inches (0.0127 cm), must be established between the outside surface of the bushing assembly and the support in order for the sleeve of the bushing assembly to be properly secured in position, the applicability of this system may also be effectively limited to a rather small tolerance range between a given bearing sleeve and support.
In the art of bearings in general, additional proposals have been made directed to establishing an interference connection between a bearing assembly and a support, even when the bearing is to be mounted in an out-of-round support member or journal. For example, U.S. Pat. No. 3,365,256 discloses a process in which several coating layers of a shearable material are applied and heat cured to an outer race of a bearing prior to mounting of the bearing within an irregularly surfaced or contoured support aperture. In accordance with this patented arrangement, the various layers can be readily sheared when the bearing is forced into the support aperture such that any gaps between an outer surface of the bearing and an inner surface of the support are filled by the shearable material. Unfortunately, this arrangement suffers from various drawbacks, including the additional material and manufacturing costs associated with applying coating after coating of extra material on either the outer surface of the bearing or, presumably, the inner surface of the support member or journal. In addition, as the shearable material is made from a different material than the bearing itself, this interface is susceptible to degradation which would result in undesirable relative rotation between the outer bearing race and the support.
Other proposed solutions to this problem of establishing a suitable fit between a bearing and a support are presented in U.S. Pat. Nos. 4,429,927 and 4,671,680. According to each of these patented arrangements, a support or journal is provided with a plurality of radially inwardly extending protrusions prior to receiving the bearing. These protrusions are able to either elastically or plastically deform to accommodate dimensional discrepancies between the components.
Unfortunately, such arrangements are not practical in the truck suspension environment due to the fact that the protrusions would have to be correspondingly formed in the spring eye portion of the leaf spring itself, which is the out-of-round portion of the entire assembly. Not only would it be extremely difficult, if not impossible, to accurately form such protrusions in a newly formed spring eye, but such an arrangement would certainly not be applicable in retrofitting existing suspension systems in need of bushing repair.
Based on the above, there exists a need in various environments for a bushing assembly including a sleeve which can be effectively, frictionally received within a support member without unduly pre-loading bearings mounted internally in the sleeve. More generally, there exists a need for a bushing assembly including a bushing member that can be effectively frictionally mounted within an internal bore of a support member, even when the bore has a size and shape within a predetermined tolerance range from an ideal size and shape. There particularly exists a need in the art of vehicle suspensions for a bushing assembly which can be mounted, without the use of a pre-loading expander, in an out-of-round or small tolerance spring eye of a leaf spring. In addition, there exists the need for a roller bushing assembly which can be installed in an out-of-round eye of a leaf spring while still establishing a tight frictional fit with the spring eye so as to present an improved roller bushing assembly having an increased useful life with minimal maintenance.